Typical propulsion systems in modern aircraft comprise a propeller, propeller blades mounted in arm bores extending from the propeller hub and a pitch change actuator for changing the pitch of the propeller blades.
The propeller blade is mounted in the arm bore for movement therein. Blade retention bearings are located circumferentially within the arm bore such to facilitate pitch change of the propeller blade. The hub is sealed and contains a specified volume of oil to lubricate the blade retention bearings. The minimum oil volume is chosen to minimize weight and ensure the arm bores are completely filled and oil distributes evenly within the cavity when acted on by centrifugal force.
The pitch change actuation device uses high pressure hydraulic fluid applied to piston located within the pitch change actuator to change blade pitch. A leak in the pitch change actuator could cause the hub to become pressurized causing high loads on the propeller components. Pitch change actuation systems are designed to place the blade in a feather position to minimize drag upon loss of hydraulic pressure. Therefore, it is more desirable to vent the hub cavity and lose pitch change capability than to pressurize the hub.
There are several prior art methods for limiting hub cavity pressure. Some systems vent the hub cavity back to a sump in the control system. If the cavity is a closed system, a pressure relief device is employed to vent the system overboard. This device can be a valve, or a component designed to fail at a predetermined pressure. FIGS. 1 and 2 illustrate prior art relief valves designed to open at a predetermined pressure. Pressure relief valves add expense and increase system weight because a mounting interface must be provided for the valve. Relief valves are also typically low flow devices, and therefore provide minimal over pressure protection in the event that there is a high flow rate leak into the hub cavity.
FIG. 1 illustrates a pressure relief device 10' wherein the cover 12' is designed to fracture releasing the spherical seal 14' to vent the hub cavity. The spherical seal 14' is located in bearing race 16' which is in fluid communication with the hub cavity. The cover 16' is mounted to an external surface 18' of the hub 20'. This device requires external mounting hardware and exhibits wide tolerances in activation pressure due to its configuration and dimensional tolerances.
FIG. 2 illustrates a second pressure relief device 22' positioned in a passage 24' located within the hub housing 26'. The pressure relief device requires a housing 28' which is attached to the hub housing 26'.
Therefore, there exists a need for a pressure relief device that provides relief for a rapid increase in hub, due to high flow rate leakage into the hub, while minimizing weight and the need for external mounting bosses and hardware.